Upgrade Gatsby to v5

[LanesGood]

Upgrading gatsby to v5 will be a required first step to permit further exploration and execution on the following issues:

• Implementing Gatsby v5 Slices API #569
• Reduce build time #351

The upgrade will also ensure future-proofing of peer dependencies; for example, currently the Gatsby-less plugin is pinned to a prior version that is dependent on v4. There may be multiple other peer deps that will need to be upgraded, but will then be more maintainable for the future.